A bending stiffness testing tool and method for sandwich structures containing complex core materials

Abstract

The present invention provides a bending stiffness test tooling and method for a sandwich structure with a complex core material. The test method includes: S1. Adjust the support span S and the loading span L of the test tooling according to preset parameters, and calculate the configuration parameters a1 and b1 according to the formula; S2. The loading tooling applies a load F downward at a preset loading speed, records the load and deflection data, generates a load-deflection curve, takes the slope of the linear part of the curve, and takes the reciprocal of the slope value as the response data c1 of the specimen; S3. Repeat the above steps to obtain a i , b i , c i ; S4. Use the formula to solve and calculate the bending stiffness D and the transverse shear stiffness U. The bending stiffness test method for the sandwich structure with a complex core material according to the present invention realizes the decoupling of the bending effect and the shear effect in the bending test, and obtains the bending stiffness and the shear stiffness of the test specimen; it can be applied to the bending performance test of the sandwich composite material with a complex core structure.

Description

Technical Field

[0001] The invention relates to the technical field of composite materials, and in particular to a bending stiffness testing tool and method for a sandwich structure containing a complex core material. Background Art

[0002] Sandwich composites are a composite structure consisting of upper and lower composite laminates and a middle core material, which can significantly improve the out-of-plane bending stiffness of the structure. The core material of traditional sandwich composites is homogeneous foam such as PVC and PMI. Its materials and structural forms are relatively simple, and the corresponding mechanical properties testing, evaluation and prediction methods are also mature.

[0003] As a structural composite material, the mechanical improvement and structural function integration of sandwich composite materials can be achieved through relevant structural design and functional addition. The structural optimization of sandwich composite materials mainly focuses on two aspects: mechanical performance optimization and structural function integration. In terms of mechanical properties, the sandwich composite structure significantly reduces weight and improves the bending stiffness of the structure due to the addition of lightweight core materials. However, the out-of-plane mechanical properties of the structure are generally weak, which is basically consistent with the core material performance. In order to improve the out-of-plane performance of the structure and the interface performance between the panel and the core material, a variety of Z-direction reinforcement methods such as three-dimensional sewing technology, Z-Pin technology, lattice reinforcement technology, etc. can strengthen the core material to a certain extent through structural design, thereby improving the application scenarios and scope of sandwich composite materials. In terms of functionality, fiber sandwich composite materials have good wave and sound transmission properties and are non-magnetic. When using sandwich composite materials to manufacture ships and their appendages, while reducing the weight of the hull, multiple functions such as acoustics and electromagnetics can be achieved by embedding multiple functional layers in the interlayer.

[0004] While sandwich composite materials achieve structural strengthening and structural function integration through enhanced design and functional design of core materials, the overall internal structure complexity of sandwich composite materials increases, which affects its structural mechanical properties, resulting in the structural performance evaluation method based on the traditional sandwich structure mechanical model based on isotropic core materials being unable to meet the evaluation requirements of sandwich composite materials with complex core material structures.

[0005] For example, Wei Kaiyao gave a detailed explanation of the bending stiffness test in the GB / T1456-2005 sandwich structure bending performance test method in "Study on Mechanical Properties of Three-dimensional Lattice Reinforced Foam Sandwich Composite Materials". From the analysis model, it can be seen that this test method only considers the bending moment, and does not consider the influence of the shear effect on the deflection curve during the bending test. Wu Xiao et al. made a detailed derivation of the bending test theoretical method in "Calculation Analysis of Four-point Bending Test of Sandwich Beams", and explained that the shear effect in the bending test is related to the support span based on theoretical mechanics; and estimated that there is a 10% shear effect when the support span is 20 times the specimen thickness. If the support span is 20 times the specimen thickness without considering the shear effect, referring to the national standard test method, the required specimen length is more than 34 times the specimen thickness. For the bending stiffness test of sandwich structures with complex core materials and sandwich structure specimens with limited length, the existing GB / T1456 sandwich structure bending performance test method does not consider the influence of the shear effect, and uses the free end outrigger position measurement, which makes the test results of sandwich structures containing complex core materials have many problems.

[0006] The influence of shear effect needs to be further considered in the test of the bending stiffness of sandwich structures with complex core materials after structural and functional reinforcement. Summary of the invention

[0007] The problem solved by the present invention is that the existing testing method does not take the shear effect into consideration when testing the bending stiffness of a sandwich structure containing a complex core material.

[0008] To solve the above problems, the present invention provides a method for testing the bending stiffness of a sandwich structure containing a complex core material, which performs a non-destructive stiffness test on the bending test specimen, decouples the bending effect and shear effect during the bending test, and improves the validity and stability of the bending stiffness test data.

[0009] The present invention provides a bending stiffness testing tool for a sandwich structure containing a complex core material, comprising:

[0010] A first support and a second support are arranged in parallel, and are used to support the sandwich sample;

[0011] A first pressing head, a loading tool is arranged above the first pressing head for applying a load to the sandwich sample; an axis of the first pressing head is perpendicular to a line between a midpoint of the first support and a midpoint of the second support;

[0012] A displacement sensor is located below the sandwich sample and is used to record disturbance data; preferably, the distance between the displacement sensor and the first support or the second support is equal.

[0013] The tooling of the present invention has a simple structure, small modification, and is easy to implement. When there is only the first pressure head, the distance between the axis of the first pressure head and the first support or the second support is equal, ensuring balanced force and high detection accuracy; at this time, L=0 represents the case where only the first pressure head is set without the second pressure head, and a three-point bending test is performed.

[0014] Preferably, the test fixture further comprises a second pressure head, and the second pressure head and the first pressure head are symmetrically arranged above the sandwich sample. At this time, if L>0, a four-point bending test is performed.

[0015] Preferably, a predetermined distance L between the second pressure head and the first pressure head is a loading span, wherein the loading span L is set to 0≤L<0.8S. This setting can ensure that the detection result is accurate and reliable.

[0016] Preferably, the first support and the second support are in the shape of round rods, and are arranged on the lower surface of the sandwich sample by rotating around their own axes. This arrangement ensures that the sandwich sample is in surface contact at the loading point and the supporting point, and the length of the contact surface is consistent with the width of the sample, such as 10-30 mm. Preferably, the extension direction of the first support and the second support is consistent with the width direction of the sandwich sample. This arrangement can ensure that the sandwich sample can rotate freely at both the loading point and the supporting point.

[0017] Preferably, the length of the sandwich sample is L1 and the thickness is H, wherein L1≥10*H. The sandwich sample is a three-layer structure, wherein the upper and lower layers are panel layers, and a core material is arranged in the middle, wherein the core material comprises a non-continuously uniformly distributed medium and / or comprises an irregular three-dimensional structure.

[0018] The present invention provides a bending stiffness testing method, which uses the bending stiffness testing tool of the sandwich structure containing a complex core material, comprising:

[0019] S1. Adjust the support span S and the loading span L of the test fixture according to preset parameters, wherein the support span S is the distance between the first support and the second support, and the loading span L is the distance between the first pressure head and the second pressure head;

[0020] According to the formula

[0021]

[0022]

[0023] Calculate configuration parameters a1 and b1;

[0024] S2, the loading tool applies the load F downward at the preset loading speed, records the load and disturbance data, and generates a load-deflection curve, takes the linear part of the curve to calculate the slope, and uses the inverse of the slope value as the response data c1 of the sample;

[0025] S3, adjust the support span S and / or the loading span L and repeat the above steps i times to obtain a2, b2, c2..., a i 、b i 、c i ;

[0026] S4. According to the formula

[0027]

[0028] Solve and calculate the bending stiffness D and transverse shear stiffness U of the sandwich specimen.

[0029] Preferably, in step S3, i≥2, and in step S4, the least square method is used for solving. In the solving process, the units of various data should be unified, preferably N-mm-MPa-s.

[0030] Preferably, the thickness of the sandwich sample is H, wherein the support span S≥3*H, and L≤0.8*S. This setting can ensure accurate and reliable detection.

[0031] Preferably, the preset loading speed is 0.5-5 mm / min, and the maximum load value F of the applied load F is max =(0.4-0.8)*F 破 , where F 破 This setting ensures that no specimen failure occurs during the testing of various configurations.

[0032] Preferably, the bending stiffness testing method of the sandwich structure containing a complex core material further comprises: S0, adjusting the testing tooling so that the support span S and the loading span L are placed in the center.

[0033] Compared with the prior art, the bending stiffness testing method for sandwich structures containing complex core materials described in the present invention has the following beneficial effects: 1) it realizes the decoupling of the bending effect and the shear effect in the bending test of sandwich composite materials, and can simultaneously obtain the bending stiffness and shear stiffness of the test sample; 2) it can be applicable to the bending performance test of sandwich composite materials with complex core material structures; 3) it improves the effective utilization rate of the bending test, and the support span can be almost consistent with the sample length. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 This is a schematic diagram of the bending stiffness test of the sandwich structure containing complex core materials according to the present invention;

[0035] Figure 2 This is a schematic diagram of the structure of the four-point bending test adopted in Example 1 of the present invention;

[0036] Figure 3This is a schematic diagram of the structure of the three-point bending test adopted in Example 2 of the present invention;

[0037] Figure 4 This is a load-disturbance relationship curve diagram in Example 2 of the present invention.

[0038] Description of reference numerals:

[0039] 1-first support; 2-second support; 31-first pressure head; 32-second pressure head; 4-loading tool; 5-sandwich specimen; 6-displacement sensor. DETAILED DESCRIPTION

[0040] In order to make the above-mentioned purposes, features and advantages of the present invention more obvious and easy to understand, the specific embodiments of the present invention are described in detail below in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Those skilled in the art can make adjustments to them as needed to adapt to specific application scenarios. It should be noted that the embodiments of the present invention and the features in the embodiments can be combined with each other without conflict.

[0041] Sandwich composite materials are widely used in the automotive, shipbuilding and aviation fields due to their lightweight bending stiffness design. Traditional sandwich composite structures can achieve structural reinforcement and / or structural functional integration of sandwich composite materials through Z-direction reinforcement technology and / or adding functional layers to their core materials. With the gradual complexity of sandwich composite structures, traditional bending stiffness test methods can no longer meet their performance measurement requirements. To this end, the applicant proposes the following technical solution:

[0042] For ease of explanation, the test device used in this application is first introduced as follows:

[0043] like Figure 1 As shown, a bending stiffness test device for a sandwich structure containing a complex core material comprises:

[0044] The base body (not shown in the figure) has a bottom surface and a top surface parallel to each other, and a first support 1 and a second support 2 are arranged in parallel on the top surface, and a predetermined distance S is defined between the first support 1 and the second support 2, and the predetermined distance S is defined as a support span;

[0045] The sandwich sample 5, i.e., the sandwich structure containing a complex core material described in the present application, is arranged on the first support 1 and the second support 2;

[0046] A first pressing head 31, a loading tool 4 is arranged above the first pressing head 31, and is used to press the sandwich sample 5; the axis of the first pressing head 31 is perpendicular to the line between the midpoint of the first support 1 and the midpoint of the second support 2, and the distance between the axis of the first pressing head 31 and the first support 1 or the second support 2 is equal;

[0047] The displacement sensor 6 is located below the sandwich sample 5 , and the distance between the displacement sensor 6 and the first support 1 or the second support 2 is equal.

[0048] Preferably, the extension direction of the first pressure head 31, the first support 1, and the second support 2 is consistent with the width direction of the sandwich sample 5. This arrangement can ensure that the sandwich sample 5 can rotate freely at both the loading point and the support point. The above structure should have sufficient rigidity to avoid significant deflection of related components during testing. The first pressure head 31 is in the shape of a round rod, which is rotatably arranged on the upper surface of the sandwich sample 5 around its own axis, and the first support 1 and the second support 2 are in the shape of a round rod, which are rotatably arranged on the lower surface of the sandwich sample 5 around their own axis; this arrangement ensures that the sandwich sample 5 is in surface contact at the loading point and the support point, and the length of the contact surface is consistent with the width of the sample, such as 10-30mm.

[0049] The loading tool 4 performs displacement loading control at a loading speed of 0.5-5 mm / min, and applies a maximum load value F max =(0.4-0.8)*F 破 , where F 破 To destroy the load value, ensure that no specimen failure occurs during the test.

[0050] The sandwich specimen 5 is a three-layer structure, wherein the upper and lower layers are panel layers, and a core material is arranged in the middle, wherein the core material contains a non-continuously uniformly distributed medium and / or contains an irregular three-dimensional structure; the length of the sandwich specimen 5 is L1, and the thickness is H, wherein L1≥10*H; wherein the support span S is taken as S>3*H, and H is the thickness of the sandwich specimen 5; the measurement accuracy of the displacement sensor 6 is not less than 1%, and the measuring point position must be located at the center of the support span, and it is prohibited to use the equipment beam displacement to represent the mid-span deflection.

[0051] As another example of the present invention, the bending stiffness testing device also includes a second pressure head 32, and the second pressure head 32 and the first pressure head 31 are symmetrically arranged above the sandwich sample 5, that is to say: the second pressure head 32 and the first pressure head 31 are at an equal distance from the midpoint of the length direction of the sandwich sample 5. There is a predetermined distance L between the second pressure head 32 and the first pressure head 31, and the predetermined distance L is defined as the loading span, wherein the loading span L is 0≤L<0.8S. When L>0, a four-point bending test is performed. Reasonable planning should be made during the test to ensure that different configuration parameters are quite different; when conditions permit, a configuration with a larger support span is preferred, and the same sandwich sample 5 is tested and calculated under multiple configuration parameter support conditions, and the number of test configurations should be no less than 2.

[0052] The sandwich specimen 5 is placed on the test fixture, and the support span S and the loading span L are adjusted according to the preset parameters. The first pressure head 31 and / or the second pressure head 32 are applied with the loading fixture 4. The load is graded to the maximum load according to the specified loading speed. The displacement sensor 6 is used to record the deflection data and the corresponding load to obtain the load-deflection curve. The linear part of the curve is selected to obtain the slope, and the inverse of the slope c is used as the response data of the specimen under the configuration condition. If there is an automatic recording instrument, continuous loading can be performed.

[0053] The present invention also provides a bending stiffness testing method, which uses the bending stiffness testing tool of the sandwich structure containing a complex core material, comprising:

[0054] S1. Adjust the support span S and the loading span L of the test fixture according to preset parameters, wherein the support span S is the distance between the first support 1 and the second support 2, and the loading span L is the distance between the first pressure head 31 and the second pressure head 32;

[0055] According to the formula

[0056]

[0057]

[0058] Calculate configuration parameters a1 and b1;

[0059] Preferably, the thickness of the sandwich sample 5 is H, wherein the support span S≥3*H, and L≤0.8*S. This setting can ensure accurate and reliable detection.

[0060] S2, the loading tool applies the load F downward at the preset loading speed, records the load and disturbance data, and generates a load-deflection curve, takes the linear part of the curve to calculate the slope, and uses the inverse of the slope value as the response data c1 of the sample;

[0061] Preferably, the preset loading speed is 0.5-5 mm / min, and the maximum load value F of the applied load F is max =(0.4-0.8)*F 破 , where F 破 This setting ensures that no specimen failure occurs during the testing of various configurations.

[0062] S3, adjust the support span S and / or the loading span L and repeat the above steps i times to obtain a2, b2, c2..., a i 、b i 、c i ;

[0063] When i=1, the solution is obtained by establishing a system of two-variable linear equations; when i≥2, the least squares method is used for solution; the units of various data in the solution process should be unified, preferably the N-mm-MPa-s unit system.

[0064] S4. According to the formula

[0065]

[0066] Solve and calculate the bending stiffness D and transverse shear stiffness U of the sandwich specimen.

[0067] Preferably, the bending stiffness test method further includes: S0, adjusting the test fixture so that the support span S and the loading span L are placed in the center.

[0068] Example 1

[0069] For typical structurally reinforced sandwich composite materials, including upper and lower panels and lattice-reinforced core materials. The panel material is glass fiber composite material, the thickness of the upper and lower panels is 2mm, the total thickness of the sample is 40mm, the width of the sample is 60mm, and the length of the sample is 1000mm.

[0070] like Figure 2 As shown, a multi-configuration four-point bending test configuration is adopted. During the test, two test configurations are used to perform non-destructive tests on the same sample. The two configurations are numbered GX-1 and GX-2. The test configuration parameters and data are shown in Table 1.

[0071] Table 1 Test parameters for different test configurations

[0072]

[0073] The configuration parameters of different test configurations were calculated according to formula a); at the same time, under each configuration test condition, a loading speed of 5 mm / min was used to obtain the load-deflection curve. According to each configuration design and load-deflection curve, the linear part of the curve was selected to calculate the slope, and the inverse of the slope value was recorded as c, which is the response data of the sample under the configuration condition. The results are shown in Table 2.

[0074] Table 2 Configuration parameters and response data of different test configurations

[0075] Test configuration a b <![CDATA[c*10 2 <!-- 5 -->]]> GX-1 11354167 125 6.54 GX-2 7333333 100 5.15

[0076] The bending stiffness of the sandwich structure specimen is obtained by solving the two-variable linear equation system using formula c) as 2.36×10 9 N*mm 2 , the transverse shear stiffness is 2.06*10 3 N.

[0077] The bending stiffness testing method for a sandwich structure containing a complex core material of the present invention performs a bending stiffness performance test on a sandwich composite material with a complex core material structure, adopts a non-free end measuring point, and adopts a multi-configuration testing technology to decouple the bending effect and the shear effect in the bending test, and finally uses a multiple linear regression fitting method to process the data, thereby realizing the testing of the bending stiffness of a sandwich composite material with a complex core material structure of limited length.

[0078] Compared with the GB / T1456 sandwich structure bending stiffness test method, the test method of the present invention fully considers the influence of the shear stiffness of the sandwich composite material on the bending test, effectively improves the sample utilization rate, ensures the validity and stability of the test data, and has good application prospects in the evaluation of the bending stiffness performance of Z-direction strengthened and / or multifunctional sandwich composite materials.

[0079] Example 2

[0080] For functional sandwich composite structure, including upper and lower panels and three layers of functional core material, the structure is complex, and the upper and lower panels are 4mm thick carbon fiber composite material. The total thickness of the sample is 70mm, the width is 120mm, and the length is 700mm. Figure 3 As shown, a reduced three-point bending test configuration is adopted, ie, L=0.

[0081] During the test, three test configurations were used to perform non-destructive tests on the same sample. The three configurations were numbered GX-400, GX-500 and GX-600. The test configuration parameters and data are shown in Table 3.

[0082] Table 3 Test parameters of different test configurations

[0083]

[0084] The configuration parameters of the three test configurations were calculated according to formula a); under different configuration test conditions, the load-deflection curves were obtained by using a displacement control loading of 0.5 mm / min. Figure 4 According to the design of each configuration and the load-deflection curve, the linear part of the curve is selected to calculate the slope, and the reciprocal of the slope value is recorded as c, which is the response data of the sample under the configuration condition. The results are shown in Table 4.

[0085] Table 4 Configuration parameters and response data of three test configurations

[0086] Test configuration a b <![CDATA[c*10 4 ]]> GX-400 1333333 100 1.18 GX-500 2604167 125 1.71 GX-600 4500000 150 2.48

[0087] The least square method was used for multiple linear regression, and the bending stiffness of the sample was calculated to be 3.49×10 10 N*mm2, shear stiffness is 1.27×10 6 N, multiple linear regression determination coefficient R 2 : 99.99%.

[0088] The bending stiffness testing method of a sandwich structure containing a complex core material described in the present invention can effectively solve the bending stiffness measurement problem caused by the complexity of the sandwich composite material structure, realize the decoupling of the bending effect and the shear effect in the bending stiffness test, and improve the test stability.

[0089] Although the present invention is disclosed as above, the present invention is not limited thereto. Any person skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the scope defined by the claims.

Claims

1. A bending stiffness test method for testing a sandwich structure containing a complex core material, characterized in that: The test fixture used comprises: a first support (1) and a second support (2) arranged in parallel, used to support a sandwich sample (5); a first pressure head (31), a loading fixture (4) arranged above the first pressure head (31), used to apply a load to the sandwich sample (5); the axis of the first pressure head (31) is perpendicular to a line between the midpoint of the first support (1) and the midpoint of the second support (2); a displacement sensor (6), located below the sandwich sample (5), used to record deflection data; The steps include: S1. adjusting the support span S and the loading span L of the bending stiffness test tool according to preset parameters, wherein the support span S is the distance between the first support (1) and the second support (2), and the loading span L is the distance between the first pressure head (31) and the second pressure head (32); According to the formula Calculate configuration parameters a1 and b1; S2, the loading tool applies a load F downward at a preset loading speed, records the load and deflection data, and generates a load-deflection curve, takes the linear part of the curve to calculate the slope, and uses the inverse of the slope value as the response data c1 of the sample; S3, adjust the support span S and / or the loading span L and repeat the above steps i times to obtain a2, b2, c2..., a i 、b i 、c i ; S4. According to the formula Solve and calculate the bending stiffness D and transverse shear stiffness U of the sandwich specimen (5).

2. The bending stiffness testing method according to claim 1, characterized in that: In step S3, i≥2, and the least square method is used to solve step S4.

3. The bending stiffness testing method according to claim 1, characterized in that: The thickness of the sandwich sample (5) is H, the support span S≥3*H, and the value range of the loading span L is 0≤L≤0.8*S.

4. The bending stiffness testing method according to claim 1, characterized in that: The preset loading speed in step S2 is 0.5-5 mm / min, and the maximum load value F of the applied load F is max =(0.4-0.8)*F 破 , where F 破 is the damage load value.

5. The bending stiffness testing method according to claim 1, characterized in that: The bending stiffness test method also includes: S0, adjusting the test fixture so that the support span S and the loading span L are placed in the center.

6. The bending stiffness testing method according to claim 1, characterized in that: The testing tool further comprises a second pressing head (32), wherein the first pressing head (31) and the second pressing head (32) are symmetrically arranged above the sandwich sample (5).

7. The bending stiffness testing method according to claim 6, characterized in that: The first support (1) and the second support (2) are in the shape of round rods and are arranged below the sandwich sample (5) so as to rotate around their own axes.

8. The bending stiffness testing method according to claim 1, characterized in that: The length and thickness of the sandwich sample (5) are L1 and H respectively, wherein L1≥10*H.

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